Future intelligent power ICs will require high-density power devices along with analog functions and VLSI logic. DMOS transistors are important in power device applications capable of handling high voltages. For such devices, one figure of merit is the current handling capability per unit area or the ON resistance per unit area. For a given voltage rating, the ON resistance per unit area may be reduced by reducing the cell area of the DMOS device.
In the field of power transistors, the combined width of the polycrystalline silicon (polysilicon) and the contact region, which forms the gate and source electrodes, respectively, is defined as the cell pitch of the device. For a DMOS power transistor, a known technique to reduce the width of the polysilicon region is by decreasing the P-well junction depth. However, minimum junction depth is dictated by the breakdown voltage required.
A conventional structure Lateral DMOS (LDMOS) device is well suited for incorporation into VLSI processes because of its simplicity. However, LDMOS devices have been considered inferior to Vertical DMOS (VDMOS) devices, and therefore have not received significant attention. Recently, a RESURF (REduced SURface Field) LDMOS device with good specific on-resistance (R.sub.sp) has been demonstrated. But that device structure is more complex and not very versatile, being limited to grounded source applications.